This invention relates to a detection device and method of detection. More particularly, the invention concerns the detection of objects which rotate or are subjected to other cyclic or repetitive motion relative to the detection device. Particularly, but not exclusively, the object is a rotating tool for use with a machine tool. Additionally the invention relates to the detection of objects which come between the device and a rotating element.
Conventionally, tools and the like have been detected using the breaking or remaking of a beam of light which may propagate directly between an aligned light transmitter and receiver e.g. as shown in EP1050368.
A problem with this arrangement is that the tool etc has to fit between the light transmitter and receiver in order to be detected. Vibrations can cause transmitter and receiver misalignment which affects the detection of the tool. Additionally the tool has to be positioned accurately which is not always possible if the exact size of the tool is not known. In this type of system a shadow is formed on the detector by the tool or other article which passes through the beam and when that shadow obscures the beam by a predetermined amount, a tool present signal is produced.
Advantageously, the tool or the like can be used to reflect light and the transmitter and receiver can be mounted together. Such systems are described in U.S. Pat. No. 5,293,048, GB2200747A and DE 34 344 61. However, the amount of light reflected by the tool is limited because tools generally have poor reflectivity, so the light transmitter must be powerful in comparison to the power of the light transmitter of the aligned system described in the paragraph above. Where the light transmitter is a laser the output power required of the laser to produce a measurable reflection from the tool may exceed current safety limits.
In U.S. Pat. No. 5,940,787 the electromagnetic radiation from the region in which a tool contacts a workpiece is detected. Depending on the condition of the tool, characteristics of the signal received by a detector between contact and retraction of the tool changes. The signal characteristics are monitored and analysed to produce a control signal. Interference from coolant and debris are filtered, but in the case of a laser system, such filtering masks certain characteristics and slows the system down making a laser system unsuitable for those situations.